For Entity Framework 5.0, What is the minimum .Net Framework version required ? either .Net 4.0 or 4.5 ?
That depends on what you mean by "required".
In order to use all features, .NET 4.5 is required. However, EF5 will run on .NET 4, but will basically function the same as EF 4.3.
EF5 will still provide some minor performance enhancements on .NET 4, and will of course provide bug fixes, but will not provide the majority of new features (like Enum support, Spatial types, etc..)
As of EF6, the new performance features were moved out of the core framework, so you can now get full benefits on .NET 4, but EF5 is still dependent upon 4.5 for the improvements.